The present invention relates generally to a projection optical system, and more particularly to a catadioptric projection system that uses a mirror to project a pattern onto a substrate, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for a so-called immersion exposure apparatus that immerses the final surface of the projection optical system and the surface of the substrate in the fluid or liquid, and exposes the substrate via the fluid.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology. As the high integration of the semiconductor device proceeds, the demand for the specification and performance of the projection optical system becomes stricter. In general, use of the exposure light having a shorter wavelength and a projection optical system having a higher numerical aperture (“NA”) are effective to high resolution. More recently, a further high NA scheme proceeds as seen in an optical system having a NA greater than 1 has been proposed, such as a so-called immersion exposure apparatus that fills a space between the final glass surface (or a lens closest to the wafer) of the projection optical system and the wafer with fluid.
When use of the exposure light having a shorter wavelength advances to such a wave range as the ArF excimer laser (having a wavelength of about 193 nm) and the F2 laser (having a wavelength of about 157 nm), the usable glass material is limited to quartz and calcium fluoride (CaF2) so as to maintain the predetermined transmittance. A dioptric optical system generally uses quartz and calcium fluoride for the exposure wavelength, for example, of 193 nm. However, these materials do not have a great difference of dispersion, and it is very difficult to correct the chromatic aberration of the optical system having a very high NA like an immersion optical system. In addition, as the NA becomes higher, an aperture of the glass material becomes large, increasing the cost of the apparatus.
Accordingly, various proposals that include a mirror in the optical system have been made to avoid problems, such as the transmittance, the chromatic aberration and a large aperture of the glass material. For example, a catadioptric projection system that combines a reflection optical system and a refractive optical system is disclosed in Japanese Patent Applications, Publication Nos. (“JPs”) 2004-205698, 8-62502 (corresponding to U.S. Pat. No. 5,861,997) and 2003-307679.
In forming a projection optical system that includes a reflection optical system for the exposure light having a short wavelength and a high NA, an optical system that has a correctable chromatic aberration, provides a sufficiently large imaging area, and is compatible with a higher NA. In particular, the NA greater than about 1.1 causes a long distance between an object (or reticle) and an image (or wafer) and a large effective diameter of the glass material, and the large size of the optical system and the high cost of the apparatus are inevitable. In addition, as the high NA advances, the tolerance to each of various aberrations, such as a distortion, becomes stricter.
JP 2004-205698 discloses a catadioptric optical system as a twice-imaging system that forms an intermediate image once. It includes a reciprocating optical system that has a concave mirror, a first imaging optical system that forms an intermediate image of the first object (or reticle), a second imaging optical system that images an intermediate image on a second object (or wafer). A first plane mirror is provided to deflect the light and optical axis near the intermediate image. The optical axis deflected by the first plane mirror is deflected approximately parallel to the reticle stage, again deflected by a second plane mirror, and imaged onto a second object. It is inevitable in the optical system disclosed in JP 2004-205698 due to the above configuration that the reticle surface, the lens, the plane mirror, and the deflected light are arranged close to each other. Therefore, the interference among the reticle surface, the reticle stage, the lens and the plane mirror becomes problematic, and it is difficult to maintain a sufficient space. In addition, FIG. 5 in JP 2004-205698 shows an immersion optical system having an NA of 1.05 and a maximum effective diameter greater than Φ300. If the high NA of 1.2 or greater is sought, the maximum effective diameter becomes very large.
FIGS. 7 and 9 of JP 8-62502 shows a catadioptric optical system as a three-time imaging system that forms an intermediate twice and has an NA between about 0.45 and about 0.5. It includes a first imaging optical system that forms a first intermediate image, a second imaging optical system that forms a second intermediate image from the first intermediate image and has a concave mirror, and a third imaging optical system that images the second intermediate image on the wafer surface. The second imaging optical system forms a reciprocating optical system having a concave mirror. In this optical system, the reticle is not parallel to the wafer. The first and second objects that are perpendicular to the gravity and parallel to each other enhance the imaging performance in a scanning exposure, and maintain the stable performance. Therefore, this optical system is unsuitable for the immersion exposure apparatus since the reticle is not parallel to the wafer, and it is difficult to retain the fluid. This optical system may need another plane mirror to arrange the reticle and the wafer in parallel. If the plane mirror is arranged near the first intermediate image, as in JP 8-62502, the optical system has a similar configuration to FIGS. 4 and 6 in JP 2003-307679.
FIG. 4 of JP 2003-307679 shows an optical system having an NA of about 0.85, arranges a plane mirror (or a reflective block) near the first and second intermediate images, accords the optical axes of the first and third imaging optical systems with each other, and arranges the reticle and wafer parallel to each other. However, this optical system becomes very large when the NA becomes 1 or greater, in particular, NA becomes 1.1 or greater, as in the immersion optical system, because the first imaging optical system from the reticle approximately to the plane mirror and the third imaging optical system approximately from the plane mirror to the wafer are arrange on the same line or optical axis. More specifically, a sum of the object-to-image distance in the first optical system and the object-to-image distance in the third optical system becomes the object-to-image distance in the entire optical system (or a distance between the reticle and the wafer). In order to prevent a large size of the optical system as the high NA advances, the refractive force of each lens should be made stronger and the aberrational correction becomes difficult. In addition, since the first imaging optical system increases a reduction ratio, the NA of the first intermediate image becomes larger by the reduction ratio than the object-side NA of the reticle. As a result, the incident angle range and the maximum incident angle upon the plane mirror increase, and pose a serious problem to the higher NA than 1. In other word, the higher NA than 1 excessively increases the incident angle range and the maximum incident angle upon the plane mirror, deteriorating the imaging performance due to the aggravated film characteristic of the plane mirror. This is true of the second intermediate image since the plane mirror is also provided near the second intermediate image.